The gluon content of the $\eta$ and ${\eta }^{\prime }$ mesons

Abstract

We analyze power-suppressed contributions to singlet pseudoscalar $\eta$ and ${\eta }^{\prime }$ meson transition form factors. These corrections stem from endpoint singularities and help improve the agreement between QCD theory and the experimental data, in particular, at low-momentum transfers. Using the CLEO data, we extract information on the profile of the ${\eta }_1$ and ${\eta }_8$ distribution amplitudes in the $SU(3{)}_F$ octet-singlet basis employing both the one-angle and the two-angle mixing schemes. In the former scheme, we find good agreement with the CLEO data, while in the second case, our approach requires non-asymptotic profiles for these mesons.

Comments: 19 pages, 6 figures consisting of 9 eps files. Needs here.sty. Typo in $bet{a}_1$ corrected. Invited plenary talk presented by the first author at Second Symposium on Threshold Meson Production in pp and pd Interactions. Extended COSY-11 collaboration meeting. Collegium Maius, Cracow, Poland, May 31-June 3, 2004